The present invention relates to a control system for a continuously variable belt-drive automatic transmission for a motor vehicle, and more particularly to a system for controlling pressure applied to control valves in a hydraulic circuit for the transmission.
A known control system for a continuously variable belt-drive transmission disclosed in U.S. Pat. No. 4,369,675 comprises an endless belt running over a drive pulley and a driven pulley. Each pulley comprises a movable conical disc which is axially moved by a fluid operated servo device so as to vary the running diameter of the belt on the pulleys in dependency on driving conditions. The system is provided with a hydraulic circuit including a pump, a line pressure control valve and a transmission ratio control valve. Each valve comprises a spool to control the oil supplied to the servo devices.
The transmission ratio control valve operates to determine the transmission ratio in accordance with the opening degree of the throttle valve of the engine and the speed of the engine. The line pressure control valve is adapted to control the line pressure in accordance with the transmission ratio and the engine speed. The line pressure is controlled to prevent the belt from slipping on the pulleys in order to transmit the output of the engine.
At idling of the engine, the transmission ratio is set at a maximum value, and the line pressure is at a maximum value. When the accelerator pedal of the vehicle is depressed, a clutch is engaged to start the vehicle. When the vehicle speed and engine speed reach set values under a driving condition, the transmission ratio starts to change (upshift). At that time if the engine speed is kept constant, the transmission ratio is automatically and continuously reduced at a speed which is determined by the line pressure, the pressure of oil supplied to the servo device of the drive pulley, and the actual transmission ratio.
In such a system, it is preferable to control the positions of the spools of the line control valve and transmission ratio control valve by control oil which is supplied to an end of each spool through a control valve. The position of the spool is controlled by controlling the quantity of oil draining from the control valve. A pressure control system is proposed, in which the control oil is obtained by raising the pressure of the drain oil discharged from the line pressure control valve. On the other hand, the line pressure is kept at a high value at low engine speed as described above. However, at low engine speed, since the discharge of the pump is small, problems such as large reduction of the control pressure or extremely high control pressure may arise at low or high engine speed. Such fluctuation of the control pressure causes malfunction of the control valve.